Electric power steering device

ABSTRACT

Provided is an electric power steering apparatus comprising a housing, a motor attached to this housing and transmitting an auxiliary steering force to a rotary shaft through a motor shaft, a worm formed on or fitted on the rotary shaft and having a gear portion formed of a metal or a resin, rolling bearings provided in the housing, disposed respectively in positions on both sides of the worm and rotatably supporting the rotary shaft, an output shaft transmitting a steering force for steering an axle and rotatably supported in a predetermined position of the housing, and a worm wheel formed on or fitted on the output shaft in a way that meshes with the worm and having a gear portion formed of the resin, wherein a pre-load mechanism for applying a pre-load acting towards the worm wheel is provided at a shaft end portion, distal from the motor, of the rotary shaft. The thus simply-constructed electric power steering apparatus is capable of eliminating an existence of a backlash and reducing tooth-butting noises without any decline of a power transmitting performance.

TECHNICAL FIELD

[0001] The present invention relates generally to an electric powersteering apparatus including a worm deceleration mechanism, and moreparticularly to an electric power steering apparatus contriving animprovement in noises caused by a backlash.

BACKGROUND ART

[0002] Generally, a play (backlash) is provided between gears in orderto obtain a smooth operation in gear meshing.

[0003] A worm wheel deceleration mechanism of the electric powersteering apparatus also requires a proper backlash. Normally, anaxis-to-axis distance between a gear housing shaft of a housing and anoutput shaft of a motor is set to the same as an axis-to-axis distance(a worm gear working radius plus a wheel gear working radius), and hencea backlash derived from a variety of working scatters in addition to thepreset backlash. This caused-by-the-scatter backlash gives a driver anuncomfortable feeling due to tooth-butting noises when a vehicle travelson a rough road, resulting in a decline of value of a commercialproduct.

[0004] A countermeasure therefor was that the tooth-butting noises arereduced by setting the backlash as small as possible while raising agear accuracy, or by a method, as disclosed in Japanese PatentApplication Laid-Open No.11-43062, of absorbing vibrations in a way thatprovides an elastic body between a worm shaft and a bearing thereof, andso forth.

[0005] Moreover, Japanese Patent Application Laid-Open No.10-281235discloses a power transmission apparatus, wherein the worm shaft issupported through the bearing in a shaft hole taking an elliptic shapethat is formed in a worm shaft housing portion and is eccentric on theside of the worm wheel, and O-ring shaped elastic member is provided ina circular groove formed concentrically in an inner peripheral surfaceof the shaft hole, this elastic member biases the bearing (the wormshaft) towards the worm wheel, thereby eliminating the backlash.

[0006] Further, when assembling the worm shaft and the worm wheel, theremight occurs a dimensional error between the worm, the worm shaft, abearing portion supporting the worm shaft, the worm wheel and a steeringshaft for supporting the worm wheel, etc. It follows that the backlashis caused at a comparatively large rate due to this dimensional errorafter being assembled. It was therefore required that the parts beassembled separately according to the accuracy thereof. Further, if ahigher output of an auxiliary steering force advances as seen over therecent years, this results in an increase in abrasions of teeth of theworm and of the worm wheel, and a drawback comes to appear, wherein theoccurrence of the backlash can not be avoided.

[0007] A known method for preventing gear butting noises derived fromthese causes is a method of eliminating the backlash by applying apre-load to the worm towards the worm wheel. For example, as disclosedin Japanese Patent Application Laid-Open Nos.2001-322554 and2001-108025, there are known methods of generating a pre-load force bycausing a deformation of an elastic body provided between an outer ringof the bearing provided at a side end of the worm and a gear housing.

[0008] Of the conventional electric power steering apparatuses disclosedin the former Publications, the apparatus disclosed in Japanese PatentApplication Laid-Open No.10-281235 has a drawback that the worm shafthousing portion is easy to abrade when the bearing moves and a problemthat the worm shaft is easy to cause an axis deviation with respect tothe motor shaft when the worm shaft is biased by the elastic member, andso on.

[0009] Generally, as far as there exists the backlash depending on acondition of a rough road, a difference between inputs from a motorvehicle and so on, a problem is that the tooth-butting noises can not becompletely muffled, and there is a necessity of reducing thetooth-butting noises for every motor vehicle.

[0010] In the conventional electric power steering apparatuses disclosedin the latter Publications, a problem is that the quantity ofdeformation of the elastic body is determined from an outside diameterof a rolling bearing that is determined based on an inside diameter of ahousing and a load, the elastic body has no alternative but minutelydeforms because of a restraint in terms of a space, besides a pre-loadforce for the worm largely changes due to a minute displacement of aworm end that is caused by a scatter in working of the gear housing andby a deflection in meshing, and it is therefore difficult to ensure thepre-load force expected.

[0011] If this pre-load force is too large, an operating force declines,which brings about deterioration in feeling when neutral of steering.Whereas if too small, the gear butting noises emit, and an essentialpurpose can not be attained.

[0012] Thus, according to the prior art, even in a case where anaxis-to-axis distance between the worm and the worm wheel changed due tothe gear abrasion, etc., the pre-load varies due to the minutedisplacement of the worm, and it was difficult to ensure the stablepre-load.

[0013] Moreover, the load and a rotational torque acting in radialdirections are applied to the elastic body provided on the outerperiphery of the bearing whenever steered, and hence, as disclosed inJapanese Patent Application Laid-Open No.2001-270448, deterioration suchas a fatigue, etc occurs in the elastic body according to a structurefor making flexural the elastic body serving as a slide bearing, and aproblem arises from this deterioration, wherein the quantity of backlashrises, and the pre-load force decreases from a permanent deformation ofthe elastic body.

[0014] The load and the rotational torque acting in the radialdirections have a great influence on the pre-load force given by theelastic body, with the result that the quantity of deformation of theelastic body augments. Therefore, if the axis-to-axis distance betweenthe worm and the worm wheel increases and if a meshing area between thegears decreases, there also arises a problem that a strength of the geardeclines this time.

[0015] An object of the present invention lies in providing asimply-structured electric power steering apparatus capable of improvingthe drawbacks to the examples of the prior art described above,eliminating an existence of the backlash and reducing tooth-buttingnoises without any decline of a power transmitting performance.

DISCLOSURE OF THE INVENTION

[0016] To accomplish the above object, according to a first invention ofthe present invention, an electric power steering apparatus with a wormgear mechanism which comprises a housing, a motor attached to thehousing and transmitting an auxiliary steering force to a rotary shaftthrough a motor shaft, a bearing provided in the housing and rotatablysupporting the rotary shaft by use of a shaft support hole, a worm beingintegral with or fitted on the rotary shaft and having a gear portionformed of a metal or a resin, an output shaft transmitting a steeringforce for steering an axle and rotatably supported in a predeterminedposition of the housing, and a worm wheel being integral with or fittedon the output shaft in a way that meshes with the worm and having a gearportion composed of a resin, the worm gear mechanism transmitting theauxiliary steering force of the motor to the output shaft, wherein themotor is installed with respect to the output shaft in such a positionthat a length which is the sum of a working radius of the worm and aworking radius of the worm wheel becomes an axis-to-axis distancebetween the output shaft and the motor shaft, and the bearing isinstalled with respect to the output shaft in such a position that anaxis-to-axis distance between the shaft support hole and the outputshaft becomes slightly smaller than an axis-to-axis distance between theoutput shaft and the motor shaft.

[0017] Further, it is preferable that the bearing be supported on therotary shaft through an elastic member, and the worm be slightly movablein an axial direction.

[0018] The apparatus being thus constructed, the rotary shaft isinstalled eccentrically towards the worm wheel from the axis of themotor, and therefore it follows that the worm shaft is pressed againstthe elastic member when assembled. This pressing force produces anelastic pre-load for pressing the worm against the worm wheel, wherebythe worm meshes with the gear portion of the worm wheel with nobacklash. Accordingly, the worm and the gear portion of the worm wheelmesh with each other by a proper frictional force, and the backlash iseliminated without any decline of a power transmitting performance.

[0019] Furthermore, according to a second invention of the presentinvention, an electric power steering apparatus with a worm gearmechanism which comprises a housing, a motor attached to the housing andtransmitting an auxiliary steering force to a rotary shaft through amotor shaft, a worm formed on or fitted on the rotary shaft and having agear portion composed of a metal or a resin, bearings provided in thehousing, disposed respectively in positions on both sides of the wormand rotatably supporting the rotary shaft, an output shaft transmittinga steering force for steering an axle and rotatably supported in apredetermined position of the housing, and a worm wheel formed on orfitted on the output shaft in a way that meshes with the worm and havinga gear portion formed of a resin, the motor installed in such a positionthat a length which is the sum of a working radius of the worm and aworking radius of the worm wheel becomes an axis-to-axis distancebetween the output shaft and the motor shaft, wherein elastic membersare disposed on both sides, in the axial direction, of the motor-sidedbearing adjacently thereto so that the rotary shaft may be slightlymovable in the axial direction within a limit of elasticity of theelastic members, the bearing positioned away from the motor is a rollingbearing, an elastic portion for biasing the worm in the meshingdirection is provided on the rotary shaft, an outer ring of the bearingis fixedly fitted in a cylindrical bearing holding member fixedly fittedin the housing, its inner ring loosely receives therein the rotatingshaft on which a cylindrical buffer member is fixedly fitted, and theelastic portion is constructed of a biasing member axially rotatablysupporting the rotary shaft, and of an elastic body receives therein,the biasing member in a position eccentric in the meshing direction ofthe worm with respect to an axis of the bearing and fixed in thevicinity of the bearing holding member.

[0020] The apparatus being thus constructed, the rotary shaft is set inthe position eccentric in the meshing direction of the worm by use ofthe elastic portion. Therefore, the rotary shaft is pressed against thebuffer member of the bearing when the worm is assembled, and an elasticpre-load for pressing the worm against the worm wheel is produced.Accordingly, the worm and the gear portion of the worm wheel mesh witheach other by the proper frictional force, and the backlash iseliminated without any decline of the power transmitting performance.

[0021] Particularly, a rate at which a volume of the elastic bodyoccupies the elastic portion is taken large, whereby an initialeccentric quantity needed for generating the pre-load can be taken largeand a spring constant of the elastic body can be decreased. Therefore,even when a configuration of the worm might change due to a scatter inworking accuracy and an abrasion of the gear, it is feasible to stablymaintain a fixed pre-load force and to effectively prevent thetooth-butting noises of the gears.

[0022] Further, the rotary shaft is movable in the axial directions, andhence, when a force is applied to the rotary shaft, the rotary shaftmoves in the axial directions within the limit of the elasticity of theelastic member, whereby the worm and the gear portion of the worm wheelmesh with each other in the proper positions to absorb an impact, andthe tooth-butting noises are thus reduced.

[0023] Moreover, the bearing receives a load and a rotational torqueacting in the meshing direction of the worm, which are generated whendriving (assisting) the apparatus, and thus controls a displacement ofthe worm. Therefore, neither a large distortion nor load occurs in theelastic body of the elastic portion, which leads to an improvement of alifetime of the elastic body.

[0024] Still further, just when the rotary shaft abuts on the shaftsupport hole of the bearing, a displacement in a meshing oppositedirection can not be made, and therefore a gear meshing area does notexcessively decrease, thereby making it possible to prevent a decline ofstrength of the gear.

[0025] Furthermore, according to a third invention of the presentinvention, an electric power steering apparatus comprises a housing, amotor attached to the housing and transmitting an auxiliary steeringforce to a rotary shaft through a motor shaft, a worm formed on orfitted on the rotary shaft and having a gear portion formed of a metalor a resin, rolling bearings provided in the housing, disposedrespectively in positions on both sides of the worm and rotatablysupporting the rotary shaft, an output shaft transmitting a steeringforce for steering an axle and rotatably supported in a predeterminedposition of the housing, and a worm wheel formed on or fitted on theoutput shaft in a way that meshes with the worm and having a gearportion formed of the resin, wherein a pre-load applying mechanism forapplying a pre-load acting towards the worm wheel is provided at theshaft side end portion, distal from the motor, of the rotary shaft.

[0026] According to the construction of the third invention, thepre-load applying mechanism sets the rotary shaft in the positioneccentric in the meshing direction of the worm, so that the worm and thegear portion of the worm wheel mesh with each other by the properfrictional force, and the backlash is eliminated without any decline ofthe power transmitting performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a sectional view of an electric power steeringapparatus, showing a first embodiment of a first invention of thepresent invention;

[0028]FIG. 2A is a partial enlarged view of a portion A in FIG. 1; FIG.2B is a sectional view of a portion of an elastic member in FIG. 1; FIG.2C shows a section taken along the line C-C in FIG. 2B;

[0029]FIG. 3 is a sectional view of an electric power steeringapparatus, showing a second embodiment of the first invention of thepresent invention;

[0030]FIG. 4 is a partial enlarged view showing a bearing portion inFIG. 3;

[0031]FIG. 5 is a sectional view of an electric power steeringapparatus, showing a third embodiment of the first invention of thepresent invention;

[0032]FIG. 6 is a sectional view of an electric power steeringapparatus, showing a fourth embodiment of the first invention of thepresent invention;

[0033]FIG. 7 is a sectional view showing a modified example of a splineconnecting portion in the electric power steering apparatus in FIG. 6;

[0034]FIG. 8 is a sectional view showing a first modified example of theelastic member in the electric power steering apparatus in FIG. 6;

[0035]FIG. 9 is a sectional view showing a second modified example ofthe elastic member in the electric power steering apparatus in FIG. 6;

[0036]FIG. 10A is a sectional view showing a third modified example ofthe elastic member in the electric power steering apparatus in FIG. 6;FIG. 10B is a sectional view taken along the line A-A in FIG. 10A;

[0037]FIG. 11 is a sectional view of an electric power steeringapparatus, showing a fifth embodiment of the first invention of thepresent invention;

[0038]FIG. 12 is a partial side view of a male spline portion of a motorshaft;

[0039]FIG. 13A is a sectional view of an electric power steeringapparatus, showing a first embodiment of a second invention of thepresent invention; FIG. 13B is a sectional view taken along the line A-Ain FIG. 13A;

[0040]FIG. 14A is a partial sectional view of an electric power steeringapparatus, showing a second embodiment of the second invention of thepresent invention; FIG. 14B is an enlarged view of a principal portionin FIG. 14A;

[0041]FIG. 15A is a partial sectional view of the electric powersteering apparatus, showing a third embodiment of the second inventionof the present invention; FIG. 15B is a sectional view taken along theline B-B in FIG. 15A;

[0042]FIG. 16A is a partial sectional view of an electric power steeringapparatus, showing a fourth embodiment of the second invention of thepresent invention; FIG. 16B is a sectional view taken along the line C-Cin FIG. 16A;

[0043]FIG. 17A is a partial sectional view of an electric power steeringapparatus, showing a fifth embodiment of the second invention of thepresent invention; FIG. 17B is a sectional view taken along the line E-Ein FIG. 17A; and

[0044]FIG. 18A is a partial sectional view of an electric power steeringapparatus, showing a sixth embodiment of the second invention of thepresent invention; FIG. 18B is a sectional view taken along the line D-Din FIG. 18A.

THE EMBODIMENTS OF THE INVENTION

[0045] An embodiment of a first invention of the present invention willhereinafter be described with reference to the drawings.

[0046]FIG. 1 is a sectional view of a configuration of an electric powersteering apparatus, showing the first embodiment of the first inventionof the present invention. FIG. 2A is a partial enlarged view of aportion A in FIG. 1. FIG. 2B is a sectional view showing a portion of anelastic member in FIG. 1.

[0047] In an electric power steering apparatus 100 in FIG. 1, anelectric motor 10, ball bearings 3 a, 3 b for rotatably supporting aworm shaft 2, an output shaft 5 of a worm wheel 4, etc., are disposed orfixed in predetermined positions in a housing 1.

[0048] The worm shaft 2 is constructed of a worm 2 a formedsubstantially on a central portion thereof, bearing support portions 2 bformed on both sides of this worm 2 a, a serrated portion 2 c formed atone end portion (at a right end portion in the Figure) of the worm shaft2, and so forth. On the other hand, a serrated hole 10 b is formedinside a motor shaft 10 a of the electric motor 10. The serrated portion2 c of the worm shaft 2 is loosely fitted in the serrated hole 10 b,whereby the worm shaft 2 is joined to the motor shaft 10 a in a way thatis movable in the axial directions but unmovable in a rotationaldirection.

[0049] As illustrated in FIGS. 1 and 2A, the bearing support portions 2b, 2 b are fitted in inner peripheral surfaces of the ball bearings 3 a,3 b. An annular groove 2 e is formed in an axially central portion ofeach bearing support portion 2 b, and an annular elastic member 6 isfitted with no clearance on this annular groove 2 e.

[0050] A diameter of an outer peripheral surface of the elastic member 6is set slightly larger than an outside diameter r of the bearing supportportion 2 b. This annular elastic member 6 includes a rubber bush 6 a asits body of which an inside-diametrical portion takes a bellows-likeshape and is fitted on a bottom peripheral surface of the annular groove2 e, and a ring-like material (for example, Teflon (registeredtrademark) material) 6 b that has a small coefficient of friction and iswelded to an outer periphery of the rubber bush 6 a in order to allowthe worm shaft 2 to move in the axial directions.

[0051] The worm wheel 4 is fixedly fitted on the output shaft 5extending in a direction orthogonal to the axial direction of the wormshaft 2 and is, the output shaft 5 being rotatably supported in apredetermined position of the housing 1 in a state of the worm wheel 4meshing with the worm 2 a, thus disposed. A gear portion 4 a of the wormwheel 4 is formed of a resin.

[0052] As illustrated in FIG. 1, in the meshing between the worm 2 a andthe worm wheel 4, let S1 (a+b=S1) be a distance which is the sum of aworking radius a of the worm 2 a and a working radius b of the wormwheel 4, and the output shaft 5 and the electric motor 10 are disposedin the housing 1 so that an axis-to-axis distance between the motorshaft 10 a of the electric motor 10 and the output shaft 5 of the wormwheel 4 becomes S1. On the other hand, the bearing 3 a on a distal sidefrom the electric motor 10 is disposed in the housing 1 in such aposition that an axis-to-axis distance S2 between a shaft support hole 3c of the bearing 3 a and the output shaft 5 becomes slightly smallerthan the axis-to-axis distance S1 between the output shaft 5 and themotor shaft 10 a. In the present embodiment, these distances S1, S2 areset such as S1=47.5 mm ad S2=47.2 mm, and a difference ΔS between theaxis-to-axis distance S1 and the axis-to-axis distance S2 is given by(S1−S2=ΔS). An optimal value of this difference ΔS is 0.1 mm through 0.5mm.

[0053] In this connection, as shown in FIG. 2, the outside diameter r ofeach of the bearing support portions 2 b rotatably supported on the ballbearings 3 a, 3 b of the worm shaft 2, is set to a dimension given by (ashaft support hole 3 c inside diameter R−2·ΔS). Along with this, athickness of the elastic member 6 is set to a value enough to permitloose fitting therein of the bearing support portion 2 b of the wormshaft 2.

[0054] In this construction, the axis of the shaft support hole 3 c ofeach of the ball bearings 3 a, 3 b is set, as described above, eccentricby ΔS towards the worm wheel 4. Therefore, when incorporated, the worm 2a and the gear portion 4 a of the worm wheel 4 mesh with each otherwithout any backlash, however, with reaction thereof, it follows thatthe worm shaft 2 (the bearing support portion 2 b) is pressed againstthe annular elastic member 6. This pressing force produces an elasticpre-load for pressing the worm 2 a against the worm wheel 4, while theworm 2 a is kept in a so-called floating state.

[0055] This pre-load force generates a friction to some extent when theworm 2 a meshes with the worm wheel 4, however, the thickness and arigidity of the annular elastic member 6 are set so that a resistanceforce thereof does not become excessively large enough to cause ahindrance to a gear performance or to such a degree that a deviation inmeshing does not occur due to an input of vibrations applied from tires.The rigidity of the elastic member 6 can be set depending on hardnessand a configuration of the rubber without any restriction.

[0056] Further, the worm shaft 2 is movable in the axial directions, andhence the worm 2 a and the gear portion 4 a of the worm wheel 4 can bemeshed with each other in their proper positions.

[0057] Thus, the inside diameter of the shaft support hole 3 c of eachof the ball bearings 3 a, 3 b is set eccentric, the diameter of the wormshaft 2 (the bearing support portion 2 b) is set comparatively small,and the annular elastic members 6 b, are provided between the worm shaft2 and the bearings 3 a, 3 b, which all cooperate to enable a pre-loadmechanism to be easily structured and the backlash to be eliminated.

[0058] According to the first embodiment of the first invention, both ofeccentric quantities of the inside diameters of the shaft support holes3 c of the bearings 3 a, 3 b are set likewise to ΔS, however, theeccentric quantity of the bearing 3 b on the proximal side to the motor10 can be set smaller than the eccentric quantity ΔS of the bearing 3 b.Namely, the distance between the axis of the bearing 3 a on the distalside from the motor and the axis of the output shaft 5, can be setlarger than the distance between the axis of the bearing 3 b on theproximal side to the motor and the axis of the output shaft 5. It isdesirable that a difference ΔS′ between the eccentric quantities be avalue of 0 through ΔS/2. This construction is more desirable in terms ofpreventing the axis deviation between the motor shaft 10 a and the wormshaft 2 by reducing the rotational resistance of the worm shaft 2.

[0059] Next, a second embodiment of the first invention of the presentinvention will be described in conjunction with the drawings.

[0060]FIG. 3 is a sectional view of a construction of the electric powersteering apparatus, showing the second embodiment of the first inventionof the present invention. FIG. 4 is an enlarged view showing a bearingportion in FIG. 3.

[0061] Throughout the following discussion, the components having thesimilar structures as those in the first embodiment of the firstinvention will be explained by use of the same numerals or symbols.

[0062] In the electric power steering apparatus 100 in FIG. 3, theelectric motor 10, the ball bearings 3 a, 3 b for rotatably supportingthe worm shaft 2, the output shaft 5 of the worm wheel 4, etc., aredisposed or fixed in predetermined positions in the housing 1.

[0063] The worm shaft 2 is constructed of the worm 2 a formedsubstantially on the central portion thereof, the bearing supportportions 2 b formed on both sides of this worm 2 a, the serrated portion2 c formed at one end portion (at a right end portion in the Figure) ofthe worm shaft 2, and so forth. On the other hand, the serrated hole 10b is formed inside the motor shaft 10 a of the electric motor 10. Theserrated portion 2 c of the worm shaft 2 is loosely fitted in theserrated hole 10 b, whereby the worm shaft 2 is joined to the motorshaft 10 a in a way that is movable in the axial directions butunmovable in the rotational direction.

[0064] As shown also in FIG. 4, annular elastic members 60 each having apredetermined thickness and predetermined elasticity are respectivelyfitted with no gap in the inner peripheral surfaces of the shaft supportholes 3 c of the ball bearings 3 a, 3 b. The annular elastic member 60is constructed of a collar portion 60 a and a cylindrical portion 60 b,wherein the cylindrical portion 60 b is fitted in the shaft support hole3 c, and the bearing support portion 2 b of the worm shaft 2 is looselyfitted in the cylindrical portion 60 b. Accordingly, the worm shaft 2 isrotatably supported by the ball bearings 3 a, 3 b through the elasticmembers 60 in these bearing support portions 2 b.

[0065] Elastic member support members 7 are fixedly fitted on the wormshaft 2, wherein these members 7 are disposed adjacent to the pair ofbearing support portions 2 b respectively on the side of the worm 2 a.These elastic member support members 7 have a function of holding thecollar portions 60 a of the elastic members 60, respectively between thebearings 3 a, 3 b and the support members 7 and allowing displacements,in the axial directions (in the right-and-left directions in theFigure), of the worm shaft 2 within a limit of the elasticity of theelastic members 60.

[0066] The worm wheel 4 is fixedly fitted onto the output shaft 5extending in the direction orthogonal to the axial direction of the wormshaft 2 and is, the output shaft 5 being rotatably supported in thepredetermined position of the housing 1 in the state of the worm wheel 4meshing with the worm 2 a, thus disposed. The gear portion 4 a of theworm wheel 4 is formed of a resin.

[0067] As illustrated in FIG. 3, in the meshing between the worm 2 a andthe worm wheel 4, let S1 (a+b=S1) be a distance which is the sum of theworking radius a of the worm 2 a and the working radius b of the wormwheel 41, and the output shaft 5 and the electric motor 10 are disposedin the housing 1 so that the axis-to-axis distance between the motorshaft 10 a of the electric motor 10 and the output shaft 5 of the wormwheel 4 becomes S1. On the other hand, the bearings 3 a, 3 b aredisposed in the housing 1 in such positions that the axis-to-axisdistance S2 between the shaft support hole 3 c thereof and the outputshaft 5 becomes slightly smaller than the axis-to-axis distance S1between the output shaft 5 and the motor shaft 10 a. The difference ΔSbetween the axis-to-axis distance S1 and the axis-to-axis distance S2 isgiven by (S1−S2=ΔS). This difference ΔS is set to an optimal value of0.1 mm through 0.5 mm.

[0068] In this connection, as shown in FIG. 4, the outside diameter r ofeach of the bearing support portions 2 b rotatably supported on the ballbearings 3 a, 3 b of the worm shaft 2, is set to a dimension given by (ashaft support hole 3 c inside diameter R−2·ΔS). Along with this, athickness of the elastic member 60 is set to a value to the extent thatthe bearing support portion 2 b of the worm shaft 2 is loosely fittedtherein.

[0069] In this construction, the axis of the shaft support hole 3 c ofeach of the ball bearings 3 a, 3 b is set, as described above, eccentricby ΔS towards the worm wheel 4. Hence, when incorporated, the worm 2 aand the gear portion 4 a of the worm wheel 4 mesh with each otherwithout any backlash, however, with the reaction thereof, it followsthat the worm shaft 2 (the bearing support portion 2 b) is pressedagainst a cylindrical portion 60 b of the annular elastic member 60.This pressing force produces an elastic pre-load for pressing the worm 2a against the worm wheel 4, and therefore the worm 2 a is kept in theso-called floating state.

[0070] This pre-load force generates the friction to some extent whenthe worm 2 a meshes with the worm wheel 4, however, the thickness and arigidity of the elastic member 60 are set so that the resistance forcethereof does not become excessively large enough to cause the hindranceto the gear performance or to such a degree that the deviation inmeshing does not occur due to the input of vibrations applied fromtires. The rigidity of the elastic member 60 can be set depending on thehardness and the configuration of the rubber without any restriction.

[0071] Further, the worm shaft 2 is movable in the axial directionswithin the limit of the elasticity of the collar portion 60 a of theelastic member 60, and hence the worm 2 a and the gear portion 4 a ofthe worm wheel 4 can be meshed with each other in their properpositions.

[0072] Thus, the inside diameter of the shaft support hole 3 c of eachof the ball bearings 3 a, 3 b is set eccentric, the diameter of the wormshaft 2 (the bearing support portion 2 b) is set comparatively small,and the elastic member 60 is provided between the worm shaft 2 and thebearings 3 a, 3 b, which all cooperate to enable a pre-load mechanism tobe easily structured and the backlash to be eliminated.

[0073] Next, a third embodiment of the first invention of the presentinvention will be described.

[0074]FIG. 5 is a sectional view of the electric power steeringapparatus, showing the third embodiment of the first invention of thepresent invention. The third embodiment of the first invention has thesame construction as the second embodiment of the first invention has,except for points that will hereinafter be explained. Therefore, thesimilar components as those in the second embodiment of the firstinvention are illustrated in a way that marks them with the samenumerals or symbols, and their repetitive explanations are omitted.

[0075] In the second embodiment of the first invention discussed above,both of the eccentric quantities of the inside diameters of the shaftsupport holes 3 c of the bearings 3 a, 3 b are set likewise to ΔS,however, according to the third embodiment of the first invention, theeccentric quantity of the bearing 3 b on the proximal side to the motor10 can be set smaller than the eccentric quantity ΔS of the bearing 3 b.Namely, in the third embodiment of the first invention illustrated inFIG. 5, a distance S3 between the axis of the bearing 3 a on the distalside from the motor and the axis of the output shaft 5, is set largerthan a distance S4 between the axis of the bearing 3 b on the proximalside to the motor and the axis of the output shaft 5. It is desirablethat a difference ΔS′ (S3−S4=ΔS′) between S3 and S4 be a value of 0through ΔS/2. This construction is more desirable in terms of preventingthe axis deviation between the motor shaft 10 a and the worm shaft 2 byreducing the rotational resistance of the worm shaft 2.

[0076] Next, a fourth embodiment of the first invention of the presentinvention will be discussed with reference to the drawings.

[0077]FIG. 6 is a sectional view of the electric power steeringapparatus, showing the fourth embodiment of the first invention of thepresent invention. FIG. 7 is a sectional view showing a modified exampleof a spline connecting portion in the electric power steering apparatusin FIG. 6. FIG. 8 is a sectional view showing a first modified exampleof the elastic member in the electric power steering apparatus in FIG.6. FIG. 9 is a sectional view showing a second modified example of theelastic member in the electric power steering apparatus in FIG. 6. FIG.10A is a sectional view showing a third modified example of the elasticmember in the electric power steering apparatus in FIG. 6. FIG. 10B is asectional view taken along the line A-A in FIG. 10A.

[0078] In the electric power steering apparatus 100 in FIG. 6, theelectric motor 10, the ball bearings 3 a, 3 b for rotatably supportingthe worm shaft 2 defined as a rotary shaft, the output shaft 5 of theworm wheel 4, etc., are disposed or fixed in predetermined positions inthe housing 1.

[0079] The worm shaft 2 is constructed of the worm 2 a formedsubstantially on the central portion thereof, and the bearing supportportions 2 b formed on both sides of this worm 2 a. A connecting member18 taking a cylindrical shape is fixedly fitted on the bearing supportportion 2 b (on the right side in the Figure) on the side of the bearing3 b proximal to the electric motor 10. A female spline portion 18 a isformed substantially in a half, in the axial direction, of an innerperipheral surface of the connecting member 18. The connecting member 18is fitted in the shaft support hole 3 c (an inner peripheral surface ofan inner ring) of the bearing 3 b so that this connecting member 18 ismovable in the axial directions.

[0080] On the other hand, a male spline portion 10 b is provided on afront end portion of the motor shaft 10 a of the electric motor 10. Thismale spline portion 10 b is loosely fitted in the female spline portion18 a of the connecting member 18, whereby the worm shaft 2 isspline-connected to the motor shaft 10 a in a state of being movable inthe axial directions but unmovable in the rotational direction. Thisspline connecting portion is set so that substantially a half of thisconnecting portion is positioned within the shaft support hole 3 c ofthe bearing 3 b. The spline connecting portion is set so that at leastthe half thereof in the axial direction, in other words, the half ormore of the spline connecting portion in the axial direction ispositioned within the shaft support hole 3 c of the bearing 3 b.

[0081] A part of an elastic member 16 taking substantially thecylindrical shape is fixedly fitted in a shaft support hole 3 d of thebearing 3 a on the distal side from the electric motor 10. This elasticmember 16 is constructed of a fitting portion 16 a, a large diametricalportion 16 b, of which an outside diameter is larger than a diameter ofthe shaft support hole 3 d, formed adjacent to the fitting portion 16 a,and a shaft support portion 16 c formed in continuation from the largediametrical portion 16 a and having an inside diameter substantiallyequal to an outside diameter of the bearing support portion 2 b. Afastening ring 11 for fixedly fitting the shaft support portion 16 c onthe bearing support portion 2 b of the worm shaft 2, is fitted on anouter peripheral portion of the shaft support portion 16 c.

[0082] This axial support portion 16 c has a function of elasticallyholding the bearing support portion 2 b of the worm shaft 2substantially in an intermediate position within the inside diameter ofthe fitting portion 16 a. The large diametrical portion 16 b has afunction of facilitating the axial movements of the worm shaft 2 by acontrivance that the shaft support portion 16 c extends and shrinks inthe axial directions when moving together with the bearing supportportion 2 b in the axial directions.

[0083] An inside diameter R1 (see also FIG. 8) of the fitting portion 16a of the elastic member 16 is set slightly larger than the outsidediameter r of the bearing support portion 2 b rotatably supported by thebearing 3 a. The outside diameter r of the bearing support portion 2 bis set to a dimension given by (a fitting portion 16 a inside diameterR1−2·ΔS). This is a value of such a degree that the bearing supportportion 2 b of the worm shaft 2 is loosely fitted in the fitting portion16 a.

[0084] On the other hand, elastic members 17 each assuming substantiallya ring-shape are disposed on both sides of the bearing 3 b proximal tothe electric motor 10 in the axial direction adjacently to the bearing 3b. These two elastic members 17 are so disposed that each is held by twopieces of ring-shaped holding members 17 a and 17 b (see also FIG. 7).The holding members 17 a positioned away from the bearing 3 b are eachfixedly fitted on the connecting member 18. The holding members 17 bpositioned on the side of the bearing 3 b are fixed to the bearing 3 band provided along the connecting member 18 a in a non-contact manner.The elastic member 17 elastically extends and shrinks in the axialdirections as the connecting member 18 moves in the axial directions,thereby allowing the worm shaft 2 to move in the axial directions withinthe limit of the elasticity thereof.

[0085] The worm wheel 4 is fixedly fitted on the output shaft 5extending in the direction orthogonal to the axial direction of the wormshaft 2. The output shaft 5 is so disposed as to be rotatably supportedin the predetermined position of the housing 1 in the state of the wormwheel 4 meshing with the worm 2 a. The gear portion 4 a of the wormwheel 4 is formed of a resin.

[0086] As illustrated in FIG. 6, in the meshing between the worm 2 a andthe worm wheel 4, let S1 (a+b=S1) be a distance which is the sum of theworking radius a of the worm 2 a and the working radius b of the wormwheel 4, and the output shaft 5 and the electric motor 10 are disposedin the housing 1 so that the axis-to-axis distance between the motorshaft 10 a of the electric motor 10 and the output shaft 5 of the wormwheel 4 becomes S1. On the other hand, the bearing 3 a on the distalside from the electric motor 10 is disposed in the housing 1 in such aposition that the axis-to-axis distance S2 between the shaft supporthole 3 d of the bearing 3 a and the output shaft 5 becomes slightlysmaller than the axis-to-axis distance S1 between the output shaft 5 andthe motor shaft 10 a. In this embodiment, the distances S1, S2 are setsuch as S1=47.5 mm, and S2=47.2 mm. It is effective that the differenceΔS (S1−S2=ΔS) between the axis-to-axis distance S1 and the axis-to-axisdistance S2 be set to an optimal value in a range of 0.1 mm through 0.5mm.

[0087] In the construction described above, the axis of the shaftsupport hole 3 d of the bearing 3 a is set, as described above,eccentric by ΔS towards the worm wheel 4. Hence, when incorporated, theworm 2 a and the gear portion 4 a of the worm wheel 4 mesh with eachother without any backlash, however, with the reaction thereof, itfollows that the worm shaft 2 (the bearing support portion 2 b) ispressed against the elastic member 6. This pressing force produces anelastic pre-load for pressing the worm 2 a against the worm wheel 4.Thus, the worm 2 a is kept in the so-called floating state.

[0088] This pre-load force generates the friction to some extent whenthe worm 2 a meshes with the worm wheel 4, however, the thickness andthe rigidity of the elastic member 16 are set so that the resistanceforce thereof does not become excessively large enough to cause thehindrance to the gear performance or to such a degree that the deviationin meshing does not occur due to the input of vibrations applied fromtires. The rigidity of this elastic member 16 can be set depending onthe hardness and the configuration of the rubber without anyrestriction.

[0089] Further, the worm shaft 2 is movable in the axial directions, andhence, when a force is applied from the tire, the worm shaft 2 moves inthe axial directions within the limit of the elasticity of the elasticmember 7, whereby the worm 2 a and the gear portion 4 a of the wormwheel 4 mesh with each other in proper positions to make it possible toabsorb an impact.

[0090] Incidentally, the worm shaft 2 is movable in directions ofmeshing between the worm 2 a and the worm wheel 4 at a portion of thebearing 3 a on the distal side from the electric motor 10. Therefore,the worm shaft 2 has a possibility of causing a hindrance to the axialmovements of the worm shaft 2 due to an axis deviation occurred in thespline connecting portion, wherein the bearing 3 b on the proximal sideto the electric motor 10 serves as a fulcrum.

[0091] According to the fourth embodiment of the first invention,however, substantially the half, in the axial direction, of the splineconnecting portion between the worm shaft 2 and the motor shaft 10 a isso set as to be positioned within the shaft support hole 3 c of thebearing 3 b. Hence, even when the worm shaft 2 slightly moves in themeshing direction of the worm 2 a, with the bearing 3 b serving as thefulcrum, a displacement of the worm shaft 2 is allowed owing to a slightplay of the spline connecting portion, and the worm shaft 2 can move inthe axial directions without wresting the spline connecting portion.

[0092] Thus, the inside diameter of the shaft support hole 3 c of theball bearing 3 a is set eccentric, the diameter of the worm shaft 2 (thebearing support portion 2 b) is set comparatively small, and the elasticmembers 16, 17 are provided between the worm shaft 2 and the bearings 3a, 3 b, which all cooperate to enable a pre-load mechanism to be easilystructured, the backlash to be completely eliminated, the impact to beabsorbed and the tooth-butting noises (rattle noises) to be restrained.

[0093] Note that the fourth embodiment of the first invention hasinvolves using the connecting member 18 for the spline connectingportion. The invention is not, however, limited to this construction andmay take such a construction that, as shown in FIG. 7, a bearing supportportion 2 b′ on the side of the bearing 3 b is molded to havesubstantially the same diameter as the outside diameter of theconnecting member 18, a connecting hole 2 f including a female splineportion 2 e for the motor shaft 10 a, and a male spline portion 10 d ofthe motor shaft 10 a is inserted into this connecting hole 2 f, thusattaining a spline connection. In this case also, the holding members 17a, 17 b are attached to the bearing support member 2 b′ with the sameconfiguration as in the embodiment of the connecting member 18 describedabove. Referring to FIG. 7, the same members as those of the apparatusin FIG. 6 are marked with the same numerals or symbols.

[0094] Next, modified examples of the elastic member 16 will bedescribed referring to FIGS. 8 to 10B inclusive.

[0095]FIG. 8 shows a first modified example of the elastic member 16.Referring to FIG. 8, the same members as those in the embodiment in FIG.6 are marked with the same numerals or symbols, and the repetitiveexplanations thereof are omitted. A cylindrical elastic member 160 isconstructed of a fitting portion 160 a (which is the same as that of theelastic member 16) serving as a portion that loosely fits therein thebearing support portion 2 b and is fixedly fitted in the shaft supporthole 3 c of the bearing 3 a, a shaft support portion 160 b adjacent tothe fitting portion 160 a, having substantially the same inside diameteras the outside diameter of the bearing support portion 2 b and receivingtherein the bearing support portion 2 b with no gap therebetween, and atruncated conic portion 160 c for integrally connecting the fittingportion 160 a and the shaft support portion 160 b. Namely, the elasticmember 160 has a configuration that an axial length of the shaft supportportion 16 c is extended instead of a configuration, the largediametrical portion 16 b is removed from the elastic member 16 describedabove, and no fastening ring 11 is provided.

[0096] A relationship between the inside diameter R1 of the fittingportion 160 a of the cylindrical elastic member 160 and the outsidediameter r of the bearing support portion 2 b is the same as in theembodiment in FIG. 6, and hence its repetitive explanation is omitted.

[0097] In this construction, the shaft support portion 160 b has afunction of elastically holding the bearing support portion 2 bsubstantially in an intermediate position within the inside diameter ofthe fitting portion 160 a. It is required that an inside diameter of theaxial support portion 160 b be set to such a dimension as to make aninner ring of the bearing 3 a rotatable as the shaft support portion 160b rotates with rotations of the bearing support portion 2 b and as tomake the bearing support portion 2 b smoothly slidable when the bearingsupport portion 2 b moves in the axial directions. It can be expectedthat this elastic member 160 exhibits the same effects as the elasticmember 16 does.

[0098]FIG. 9 shows a second modified example of the elastic member 16.Referring to FIG. 9, the same members as those in the embodiment in FIG.6 are marked with the same numerals or symbols, and the repetitiveexplanations thereof are omitted. A cylindrical elastic member 161 isconstructed of an shaft support portion 161 a having substantially thesame inside diameter as the outside diameter of the bearing supportportion 2 b and fitting therein the bearing support portion 2 b with nogap, and a protruded portion 161 b formed as a large diametrical portionhaving a small wall thickness formed integrally with an intermediateportion, in the axial direction, of the shaft support portion 161 a, theprotruded portion 161 b being fixedly fitted in the shaft support hole 3c of the bearing 3 a. It is required that an inside diameter of theshaft support portion 161 a be set to a dimension that makes the bearingsupport portion 2 b smoothly slidable when the bearing support portion 2b moves in the axial directions. A ring-shaped stopper 19 is fixedlyfitted on a left end portion, as viewed in the Figure, of the bearingsupport portion 2 b.

[0099] In this construction, the shaft support portion 161 a permits thebearing support portion 2 b to move in the axial direction (away ortowards from the motor 10) but the stopper 19 restricts. When thebearing support portion 2 b moves in the meshing direction of the worm 2a, the protruded portion 161 b is compressed in a direction that pressesthe inner peripheral surface of the shaft support hole 3 c, therebyelastically allowing the movement.

[0100] Next, a third modified example of the elastic member 16 is shownin FIG. 10A. Referring to FIG. 10A, the same members as those in theembodiment in FIG. 6 are marked with the same numerals or symbols, andtheir repetitive explanations are omitted. A cylindrical elastic member162 is fixedly fitted in the holding member 162 a, and the holdingmember 162 a is fixedly fitted in the shaft support hole 3 d of thebearing 3 a. An inside diameter of the elastic member 162 issubstantially equal to the outside diameter of the bearing supportportion 2 b, and is set to a dimension that makes the bearing supportportion 2 b smoothly slidable when the bearing support member 2 b movesin the axial directions.

[0101] A multiplicity of grooves 162 b are, as illustrated in FIG. 10B,formed along an axial direction on an inner peripheral surface of theelastic member 162, whereby the elastic member 162 is easy to becompressed from inside its inner peripheral surface. When the bearingsupport portion 2 b moves in the meshing direction of the worm 2 a, thebearing support portion 2 b presses the inner peripheral surface (formedwith the multiplicity of grooves 162 b) of the elastic member 162 andthus compresses the elastic member 162 in this direction, therebyelastically allowing the movement.

[0102] The ring-shaped stopper 19 is fixedly fitted on the left endportion, as viewed in the Figure, of the bearing support portion 2 b.The elastic member 162 permits the bearing support portion 2 b to movein the axial direction (away from or towards the motor 10) but thestopper 19 restricts the movement in the axial direction.

[0103] As discussed above, according to the fourth embodiment of thefirst invention of the present invention, the motor-sided bearing hasthe elastic members disposed on both sides in the axial directionadjacently thereto, whereby the worm is slightly movable in the axialdirection. Besides, the bearing on the distal side from the motorsupports the rotary shaft (the worm shaft) through the elastic member,and this rotary shaft is slightly movable in the meshing direction ofthe worm. Therefore, with the simple construction, the backlash can becompletely eliminated. Further, when the force is applied to the rotaryshaft, the rotary shaft moves in the axial directions within the limitof the elasticity of the elastic member, with the result that the wormand the gear portion of the worm wheel mesh with each other in theproper positions to absorb the impact. It is therefore possible toreduce the tooth-butting noises without any decline of a transmissionperformance of an auxiliary steering force.

[0104] Moreover, in the fourth embodiment of the first invention, atleast the half, in the axial direction, of the spline connecting portionbetween the rotary shaft and the motor shaft is positioned within theshaft support hole of the bearing. Therefore, even when the rotary shaftslightly moves in the meshing direction of the worm, as the motor-sidedbearing serves as the fulcrum, the rotary shaft can move in the axialdirection without wresting the spline connecting portion.

[0105] Furthermore, the pre-load force of the worm can be adjusteddepending on the material and the configuration of the elastic member,and hence dimensional accuracies are not excessively required of therotary shaft and the bearing as well, thereby facilitating thedimensional control.

[0106] Next, a fifth embodiment of the first invention of the presentinvention will be explained in conjunction with FIGS. 11 and 12. FIG. 11is a sectional view of the electric power steering apparatus, showingthe fifth embodiment of the first invention. FIG. 12 is a partial sideview of the male spline portion of the motor shaft.

[0107] In the fifth embodiment of the first invention, the componentshaving the same structures as those in the aforementioned embodiments ofthe first invention are shown with the same numerals or symbols.

[0108] In the electric power steering apparatus 100 in FIG. 11, theelectric motor 10, the ball bearings 3 a, 3 b for rotatably supportingthe worm shaft 2 defined as the rotary shaft, the output shaft 5 of theworm wheel 4, etc., are disposed or fixed in predetermined positions inthe housing 1.

[0109] The worm shaft 2 is constructed of the worm 2 a formed on thecentral portion thereof, and the bearing support portions 2 b, 2 b′formed on both sides of this worm 2 a. A relationship between thebearing portion 2 b′ proximal to the electric motor 10 and the motorshaft 10 a of the electric motor 10 is similar to what is shown in FIG.7. Namely, the bearing portion 2 b′ is formed with the connecting hole 2f opened toward the motor side and having the female spline portion 2 e.An end of the motor shaft 10 a is formed with a male spline portion 10d′. The female spline portion 2 e is spline-connected to the male splineportion 10 d′.

[0110] In the fifth embodiment of the first invention, the male splineportion 10 d′ of the motor shaft 10 a takes, as illustrated in FIG. 12,a drum-like shape whose diameter is small at both of its ends with anarrow key width but is large at its central portion in the axialdirection. The male spline portion 10 d′ is fitted into the bearingportion 2 b′ of the worm shaft at a central portion of the ball bearing3 b. A diameter of the female spline portion of the bearing portion 2 b′remains unchanged in the axial direction.

[0111] In the fifth embodiment of the first invention, a structurebetween the ball bearing 3 a on the distal side from the motor 10, thebearing support portion 2 e of the worm shaft 2 and the cylindricalelastic member 160 interposed therebetween, is the same as what is shownin FIG. 8.

[0112] In the fifth embodiment of the first invention, structures otherthan the above-mentioned are the same as those in the embodimentsdiscussed above, and hence their repetitive explanations are omitted.

[0113] In the fifth embodiment of the first invention, when the electricmotor 10 generates an assist torque, the worm receives the force in sucha direction as to get away from the wheel. At this time, the worm istilted about the central portion, serving as a fulcrum, of the bearingon the proximal side to the motor, however, the spline portions do notinterfere with each other, and neither the operating force nor a feelingat steering neutral time is deteriorated. Further, in the case where theworm moves in the axial direction, the interference between the splineportions does not occur.

[0114] Moreover, the end portion of the male spline portion 10 d′ of themotor shaft 10 a is small of its diameter, and hence when the apparatusis assembled, the male spline portion 10 d′ is easy to fit into the malespline portion 2 e, thereby improving the operability.

[0115] Next, embodiments of a second invention of the present inventionwill be discussed with reference to the drawings.

[0116]FIG. 13A is a sectional view of the electric power steeringapparatus, showing a first embodiment of the second invention of thepresent invention. FIG. 13B is a sectional view taken along the line A-Ain FIG. 13A. FIG. 14A is a sectional view of the electric power steeringapparatus, showing a second embodiment of the second invention of thepresent invention. FIG. 14B is an enlarged view of a principal portionin FIG. 14A. FIG. 15A is a sectional view of the electric power steeringapparatus, showing a third embodiment of the second invention of thepresent invention. FIG. 15B is a sectional view taken along the line B-Bin FIG. 15A. FIG. 16A is a sectional view of the electric power steeringapparatus, showing a fourth embodiment of the second invention of thepresent invention. FIG. 16B is a sectional view taken along the line C-Cin FIG. 16A. FIG. 17A is a sectional view of the electric power steeringapparatus, showing a fifth embodiment of the second invention of thepresent invention. FIG. 17B is a sectional view taken along the line E-Ein FIG. 17A. FIG. 18A is a sectional view of the electric power steeringapparatus, showing a sixth embodiment of the second invention of thepresent invention. FIG. 18B is a sectional view taken along the line D-Din FIG. 18A.

[0117] In the electric power steering apparatus 100 in FIGS. 13A and 13Bshowing the first embodiment of the second invention of the presentinvention, an electric motor 101, ball bearings 103 a, 103 c forrotatably supporting a worm shaft 102 defined as a rotary shaft, anelastic portion 103 b, an output shaft 105 of a worm wheel 104, etc.,are disposed or fixed in predetermined positions in a housing 101.

[0118] The worm shaft 102 is constructed of a worm 102 a formedsubstantially on the central portion thereof, and bearing supportportions 102 b formed on both sides of the worm 102 a. A cylindricalconnecting member 108 is fixedly fitted on the bearing support portion102 b (the right side in the Figure) on the side of the ball bearing 103c proximal to the electric motor 110. A female spline portion 108 a isformed substantially in a half, in the axial direction, of an innerperipheral surface of the connecting member 108. The connecting member108 is fitted in a shaft support hole 103 d (an inner peripheral surfaceof an inner ring) of the ball bearing 103 c so that the connectingmember 108 is movable in the axial directions.

[0119] On the other hand, a male spline portion 110 b is provided on anend portion of the motor shaft 110 a of the electric motor 110. Thismale spline portion 110 b is loosely fitted in the female spline portion108 a of the connecting member 108, whereby the worm shaft 102 isspline-connected to the motor shaft 110 a in a state of being movable inthe axial directions but unmovable in the rotational direction. Thisspline connecting portion is set so that substantially a half of theconnecting portion in the axial direction is positioned within the shaftsupport hole 103 d of the bearing 103 c. The connecting member 108 maybe formed integrally with the worm 102 a.

[0120] On the opposite side to the motor, the ball bearing 103 a and anelastic portion 103 b are disposed side by side in the axial direction.The bearing 103 a has a function as a rolling bearing, and an outer ringthereof is fixedly fitted in a bearing ring 114 defined as a cylindricalbearing holding member fixedly fitted in the housing 101. A cylindricalbuffer member 106 formed of a resin, etc., which is fixedly fitted onthe bearing support portion 102 b of the worm shaft 102, is looselyfitted in an inner ring of the bearing 103 a.

[0121] The elastic portion 103 b is constructed of a biasing member 112having an inside diameter substantially equal to an outside diameter ofthe bearing support portion 102 b and being formed of resin, and anelastic body 113 fixedly fitting therein the biasing member 112 andfitting rein, fixedly fitted in the bearing ring 114 and formed of arubber, etc. A position in which the elastic body 113 receives thereinthe biasing member 112 is far eccentric in the meshing direction of theworm 102 a. When incorporating the worm shaft 102 into the ball bearing103 a and the elastic portion 103 b, however, the worm shaft 102 becomessubstantially concentric with the bearing 103 a, and hence the elasticbody 113 deforms in a direction opposite to the meshing direction of theworm 102 a, thereby producing a pre-load force in the meshing direction.

[0122] An outside diameter of the buffer member 106 is set slightlysmaller than an inside diameter of the inner ring of the bearing 103 a.Let ΔS be a displacement quantity of the worm 102 a in the meshingdirection for eliminating the backlash, and an outside diameter of thebuffer member 106 is set to a dimension given by (an inner ring insidediameter−2·ΔS). This is a value of such a degree that the buffer member106 is loosely fitted into the inner ring.

[0123] On the other hand, elastic members 107 each assumingsubstantially a ring-shape are disposed on both sides of the bearing 103c proximal to the electric motor 110 in the axial direction adjacentlyto the bearing 103 c. These two elastic members 107,107 are disposed inthe form of each being held by two pieces of ring-shaped holding members107 a and 107 b. The holding members 107 a positioned away from thebearing 103 c are each fixedly fitted on the connecting member 108. Theholding members 107 b abutting on the bearing 103 c are fixed to thebearing 103 b and provided along the connecting member 108 in anon-contact manner. The elastic member 107 elastically extends andshrinks in the axial directions as the connecting member 108 moves inthe axial directions, thereby allowing the worm shaft 102 to move in theaxial directions within the limit of the elasticity thereof.

[0124] The worm wheel 104 is fixedly fitted on the output shaft 105extending in the direction orthogonal to the axial direction of the wormshaft 102. The output shaft 105 is so disposed as to be rotatablysupported in a predetermined position of the housing 101 in a state ofthe worm wheel 104 meshing with the worm 102 a. A gear portion 104 a ofthe worm wheel 104 is formed of a resin.

[0125] As illustrated in FIG. 13A, in the meshing between the worm 102 aand the worm wheel 104, let S (a+b=S) be a distance which is the sum ofa working radius a of the worm 102 a and a working radius b of the wormwheel 104, and the output shaft 105, the bearing 103 c and the electricmotor 110 are disposed in the housing 101 so that an axis-to-axisdistance between the motor shaft 110 a of the electric motor 110 and theoutput shaft 105 of the worm wheel 104 and an axis-to-axis distancebetween the bearing 103 c and the output shaft 105 become S. In thisembodiment, the distance S is set such as S=47.5 mm. It is effectivethat the displacement quantity ΔS of the worm 102 a in the meshingdirection for eliminating the backlash be set to an optimal value in arange of 0.1 mm through 0.5 mm.

[0126] In the construction described above, a position in which thebiasing member 112 is fitted in the elastic body 113 of the elasticportion 103 b, is eccentric in the meshing direction of the worm 102 a,and hence, when assembling the worm shaft 102 into the bearing 103 a,the elastic body 113 deforms to produce a pre-load force in the meshingdirection of the worm 102 a. Then, the worm 102 a and the gear portion104 a of the worm wheel 104 mesh with each other without the backlash.Thus, the worm 102 a is kept in the so-called floating state.

[0127] Particularly, a rate at which a volume of the elastic body 113occupies the elastic portion 103 b is taken large, whereby an initialeccentric quantity needed for generating the pre-load can be taken largeand a spring constant of the elastic body 113 can be decreased.Therefore, even when a configuration of the worm 102 a might change dueto a scatter in working accuracy and an abrasion of the gear, it isfeasible to stably maintain a fixed pre-load force and to effectivelyprevent the tooth-butting noises of the gears.

[0128] This pre-load force generates the friction to some extent whenthe worm 102 a meshes with the worm wheel 104, however, a thickness anda rigidity of the elastic body 113 are set so that a resistance forcethereof does not become excessively large enough to cause a hindrance toa gear performance or to such a degree that a deviation in meshing doesnot occur due to an input of vibrations applied from tires. The rigidityof this elastic body 113 can be set depending on hardness and aconfiguration of the rubber without any restriction.

[0129] Further, the worm shaft 102 is movable in the axial directions,and hence, when a force is applied from the tire, the worm shaft 102moves in the axial directions within a limit of elasticity of theelastic member 107, whereby the worm 102 a and the gear portion 104 a ofthe worm wheel 104 mesh with each other in proper positions to make itpossible to absorb an impact.

[0130] In the portion of the ball bearing 103 a, even in a case where apressure is exerted on the buffer member 106 due to an abruptdisplacement of the worm 102 a in the meshing direction thereof, thebuffer member 106 absorbs vibrations, thereby preventing an emission ofnoises of collision. Further, the buffer member 106 is formed of theresin and is therefore effective in reducing a friction caused when theworm shaft 102 moves in the axial directions.

[0131] Moreover, in the ball bearing 103 a, the minute gap ΔS isprovided between the inner ring and the buffer member 106 of the wormshaft 102, and hence there is absorbed a change in the axis-to-axisdistance between the worm shaft 102 and the output shaft 105 due to thescatter in the working accuracy and to the meshing, whereby the stableoperation can be ensured.

[0132] Further, the ball bearing 103 a receives a load and a rotationaltorque acting in the meshing direction of the worm 102 a, which aregenerated when driving (when assisting) the apparatus, and controls thedisplacement of the worm 102 a. Therefore, neither a large distortionnor load occurs in the elastic body 113 of the elastic portion 103 b,thereby a durability of the elastic body 113 to be improved.

[0133] Thus, the fitting position of the bearing support portion 102 binto the elastic body 113 of the elastic portion 103 b is set eccentric,the outside diameter of the buffer member 106 is set comparativelysmaller than the inside diameter of the inner ring of the bearing 103 a,and the elastic member 107 is provided between the worm shaft 102 andthe bearing 103 c, which all cooperate to enable a pre-load mechanism tobe easily structured, the backlash to be completely eliminated, theimpact to be absorbed and the tooth-butting noises (rattle noises) to berestrained.

[0134] Next, a second embodiment of the second invention of the presentinvention will be explained referring to FIGS. 14A and 14B. The secondembodiment of the second invention is substantially the same as thefirst embodiment of the second invention discussed above, wherein thesame members are marked with the same numerals or symbols, and theirrepetitive explanations are omitted. A different point is that thebuffer member 106 is fixedly fitted in the inner ring of the firstbearing 103 a. The buffer member 106 is bonded to a cylinder 115 whosewall is thin, and the cylinder 115 is fixedly fitted in the inner ring.In this construction also, the same effects as the first embodiment ofthe second invention exhibits can be expected. Further, this buffermember 106 may also be fixed to the inner ring through no intermediaryof the cylinder 115.

[0135] Subsequently, a third embodiment of the second invention of thepresent invention will be described with reference to FIGS. 15A and 15B.The third embodiment of the second invention is substantially the sameas the first embodiment of the second invention discussed above, whereinthe same members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that therubber-made elastic body 113 of the elastic portion 103 b is replacedwith a resin-made elastic body 116, and, as illustrated in FIG. 15B, aplurality of holes 116 a are formed along a periphery of a biasingmember 112.

[0136] In this construction, the elastic body 116 functions as anelastic body capable of elastically deforming in the meshing directionof the worm 102 a owing to the plurality of holes 116 a. The thirdembodiment can be also expected to exhibit the same effects as those inthe first embodiment of the second invention, and may require a lessnumber of parts, which contributes to reduce the costs.

[0137] Next, a fourth embodiment of the second invention of the presentinvention will be discussed with reference to FIGS. 16A and 16B. Thefourth embodiment of the second invention is substantially the same asthe first embodiment of the second invention discussed above, whereinthe same members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that a torsionspring 117 is adopted as a substitute for the elastic body 113 of theelastic portion 103 b. In this case, the elastic portion 103 b isconstructed of a biasing member 112 for rotatably supporting the bearingsupport portion 102 b of the worm shaft 102, the torsion spring 117wound on an outer peripheral portion of the biasing member 112, and alatching member 118 that latches both side ends of the torsion spring117 in order to support the biasing member 112 in a position eccentricin the meshing direction of the worm 102 a and is fixedly fitted in thebearing ring 114. As shown in FIG. 16B, the torsion spring 117 is openat its both side ends in an initial state but resiliently closes whenincorporated into the latching member 118, and a biasing force generatedat this time produces a pre-load force acting in the meshing directionin the worm 102 a. In this construction also, the same effects as thefirst embodiment of the second invention has can be expected.

[0138] Next, a fifth embodiment of the second invention of the presentinvention will be explained referring to FIGS. 17A and 17B. The fifthembodiment of the second invention is substantially the same as thefirst embodiment of the second invention discussed above, wherein thesame members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that thetorsion spring 117 is adopted as a substitute for the elastic body 113of the elastic portion 103 b. In this case, the elastic portion 103 b isconstructed of the biasing member 112 rotatably supporting the bearingsupport portion 102 b of the worm shaft 102, the torsion spring 117wound on the outer peripheral portion of the biasing member 112, and thelatching member 118 that latches both ends of the torsion spring 117 inorder to support the biasing member 112 in a position eccentric in themeshing direction of the worm 102 a and is fixedly fitted in the bearingring 114. The biasing member 112 is fitted at the its center thereof onthe worm shaft 102 b and generates a pre-load with respect to the worm102 a by dint of a rewinding force of the torsion spring 117 wound onalong the outer periphery thereof concentrically with a hole thereof. Acontact portion between the biasing member 112 and the torsion spring117 is opposed to the wheel 104 and is well short for an inner peripheryof the torsion spring 117, the structure being such that the pre-loadgenerated by the torsion spring 117 can be efficiently transferred tothe biasing member 112. Further, the latching member 118 is fixedlyfitted in the housing 101 and fixed by the bearing ring 114. Thelatching member 118 may be formed either by press working or of a resin,etc.

[0139] As illustrated in FIG. 17B, the torsion spring 117 has two piecesof hooks 117 a provided at both ends and disposed respectively inpositions that are shifted by 180° in phase with respect to a windingcenter of the torsion spring 117 in the initial state, wherein thesehooks 117 a are latched by projections of the latching member 118.Moreover, the torsion spring 117 is given a torsional torque beforehand.With this contrivance, the biasing member 112 is, before assembling theworm 102 a, held by the projections 118 a of the latching members inpositions slightly shifted with respect to the elastic portion, and thebiasing member 112 is displaced by assembling the worm 102 a with theresult that the pre-load force is to be produced for the worm 102 a.Therefore, any processes specializing in adjusting and in giving thepre-load are not required, and a built-in characteristic is improved.Moreover, the predetermined pre-load force can be ensured even bysetting the spring constant comparatively low. In this constructionalso, the same effects as those in the first embodiment of the secondinvention can be expected.

[0140] Moreover, a sixth embodiment of the second invention of thepresent invention will be explained with reference to FIGS. 18A and 18B.The sixth embodiment of the second invention is substantially the sameas the first embodiment of the second invention discussed above, whereinthe same members are marked with the same numerals or symbols, and theirrepetitive descriptions are omitted. A different point is that a spiralspring 119 is adopted as a substitute for the elastic body 113 of theelastic portion 103 b. One end of the spiral spring 119 is fixed to theouter peripheral portion of the biasing member 112 rotatably supportingthe bearing support portion 102 b of the worm shaft 102, and the otherend thereof is fixed to the bearing ring 114. The biasing member 112 iseccentric in the meshing direction of the worm 102 a with respect to theaxis of the bearing ring 114, and, when assembling the worm shaft 102into the ball bearing 103 a, the spiral spring 119 deforms to producethe pre-load acting in the meshing direction in the worm 102 a. In thisconstruction also, same effects as those in the first embodiment of thesecond invention can be expected.

INDUSTRIAL APPLICABILITY

[0141] As described above, according to the first invention of thepresent invention, the motor is installed with respect to the outputshaft in such a position that the length which is the sum of the workingradius of the worm and the working radius of the worm wheel becomes theaxis-to-axis distance between the output shaft and the motor shaft. Thebearing is installed with respect to the output shaft in such a positionthat the axis-to-axis distance between the shaft support hole and theoutput shaft becomes slightly smaller than the axis-to-axis distancebetween the output shaft and the motor shaft. The elastic member havingthe predetermined elasticity and thickness is interposed between theinner peripheral surface of the shaft support hole of the bearing andthe rotary shaft. The outside diameter of the portion of the rotaryshaft on which the elastic member is fitted, is set smaller than theinside diameter of the shaft support hole to such a degree that theelastic member gives the proper biasing force towards the worm wheel ofthe worm. Therefore, with the simple construction, the backlash can beeliminated, and the tooth-butting noises can be reduced without anydecline of the transmitting performance of the auxiliary steering force.

[0142] Further, the pre-load force can be adjusted based on the materialand the configuration of the elastic member, and hence the dimensionalaccuracies are not excessively required of the rotary shaft and thebearing as well, thereby facilitating the dimensional control.

[0143] Moreover, according to the second invention of the presentinvention, with respect to the motor-sided bearing, the elastic membersare disposed on both sides in the axial direction thereof and adjacentlythereto, and the worm is set slightly movable in the axial directions.Besides, the bearing on the distal side from the motor is the rollingbearing, and the elastic portion for biasing the worm in the meshingdirection is provided on the rotary shaft. The outer ring of the bearingis fixedly fitted in the cylindrical bearing support member fixedlyfitted in the housing, and the inner ring of this bearing looselyreceives therein the cylindrical buffer member fixedly fitted on therotary shaft. The elastic portion is constructed of the biasing memberfor rotatably supporting the rotary shaft and of the elastic bodyreceiving therein this biasing member in the position that is eccentricin the meshing direction of the worm with respect to the axis of thebearing, this elastic body being fixed in the vicinity of the bearingsupport member. Therefore, the worm has the pre-load force produced inthe meshing direction thereof, and, with the simple construction, thebacklash can be eliminated completely.

[0144] Particularly, the rate at which the volume of the elastic bodyoccupies the elastic portion is taken large, whereby the initialeccentric quantity needed for generating the pre-load can be taken largeand the spring constant of the elastic body can be decreased. Therefore,even when the configuration of the worm might change due to the scatterin working accuracy and the abrasion of the gear, it is feasible tostably maintain the fixed pre-load force and to effectively prevent thetooth-butting noises of the gears.

[0145] Further, when the force is applied to the rotary shaft, therotary shaft moves in the axial directions within the limit of theelasticity of the elastic member, with the result that the worm and thegear portion of the worm wheel mesh with each other in the properpositions to absorb the impact. It is therefore possible to reduce thetooth-butting noises without any decline of the transmission performanceof the auxiliary steering force.

[0146] Moreover, the bearing receives the load and the rotational torqueacting in the meshing direction of the worm, which are generated whendriving the apparatus, thus controlling the displacement of the worm.Therefore, neither the large distortion nor load occurs in the elasticbody of the elastic portion, which leads to an improvement of a lifetimeof the elastic body.

[0147] Furthermore, according to the third invention of the presentinvention, the electric power steering apparatus includes the housing,the motor attached to this housing and transmitting the auxiliarysteering force to the rotary shaft through the motor shaft, the wormformed on or fitted on this rotary shaft and having the gear portionformed of the metal or the resin, the rolling bearings provided in thehousing, disposed respectively in the positions on both sides of theworm and rotatably supporting the rotary shaft, the output shafttransmitting the steering force for steering an axle and rotatablysupported in the predetermined position of the housing, and the wormwheel formed on or fitted on the output shaft in a way that meshes withthe worm and having the gear portion formed of the resin, wherein thepre-load mechanism for applying the pre-load acting towards the wormwheel is provided at the shaft side end portion, distal from the motor,of the rotary shaft. It is possible to provide the thussimply-constructed electric power steering apparatus capable ofeliminating an existence of the backlash and reducing the tooth-buttingnoises without any decline of the power transmitting performance.

1. An electric power steering apparatus comprising: a housing; a motorattached to said housing and transmitting an auxiliary steering force toa rotary shaft through a motor shaft; a bearing provided in said housingand rotatably supporting said rotary shaft by a shaft support hole; aworm rotating together with said rotary shaft and having a gear portionformed of a metal or a resin; an output shaft transmitting a steeringforce for steering an axle and rotatably supported in a predeterminedposition of said housing; a worm wheel rotating together with saidoutput shaft in a way that meshes with said worm and having a gearportion formed of a resin; and a worm gear mechanism transmitting theauxiliary steering force of said motor to said output shaft, whereinsaid motor is installed with respect to said output shaft in such aposition that a length which is the sum of a working radius of said wormand a working radius of said worm wheel becomes an axis-to-axis distancebetween said output shaft and said motor shaft, and said bearing isinstalled with respect to said output shaft in such a position that anaxis-to-axis distance between said shaft support hole and said outputshaft becomes slightly smaller than an axis-to-axis distance betweensaid output shaft and said motor shaft.
 2. An electric power steeringapparatus according to claim 1, wherein said bearing is supported onsaid rotary shaft through an elastic member, and said worm is slightlymovable in an axial direction.
 3. An electric power steering apparatusaccording to claim 1, wherein a difference between an axis-to-axisdistance between said motor shaft and said output shaft and anaxis-to-axis distance between said shaft support hole and said outputshaft, is set to 0.1 through 0.5 mm.
 4. An electric power steeringapparatus according to claim 1, wherein said bearing is constructed of afirst bearing positioned in proximity to said motor and a second bearingpositioned away from said motor, and an axis-to-axis distance betweensaid first bearing and said rotary shaft is larger than an axis-to-axisdistance between said second bearing and said rotary shaft.
 5. Anelectric power steering apparatus according to claim 1, wherein saidbearing is constructed of a first bearing positioned in proximity tosaid motor and a second bearing positioned away from said motor, elasticmembers are disposed on both sides, in the axial direction, of saidfirst bearing adjacently to said first bearing, so that said rotaryshaft is set slightly movable in the axial direction within a limit ofelasticity of said elastic member, and said second bearing supports saidrotary shaft through said elastic members, so that said rotary shaft isset slightly movable in a meshing direction of said worm.
 6. An electricpower steering apparatus according to claim 5, wherein at least a half,in the axial direction, of a spline connecting portion between saidrotary shaft and said motor shaft is so set as to be positioned withinsaid shaft support hole of said motor-sided bearing.
 7. An electricpower steering apparatus according to claim 5, wherein said elasticmember of said motor-sided bearing or said rotary shaft in the vicinityof bearing positioned away from said motor or both of said elasticmember and said rotary shaft, is or are provided with a stopper forcontrolling a quantity of movements of said rotary shaft in the axialdirections.
 8. An electric power steering apparatus according to claim5, wherein said rotary shaft and said motor shaft are spline-connectedto each other, and said rotary shaft is so formed as to be swayableabout the spline connecting portion serving as a fulcrum.
 9. An electricpower steering apparatus according to claim 8, wherein said motor shaftis formed with a male spline portion, said rotary shaft is formed with afemale spline portion, said motor shaft and said rotary shaft arespline-connected to each other, and said male spline portion is formedin a drum-like shape so that its diameter is small at both side endsthereof in the axial direction and is large at a central portionthereof.
 10. An electric power steering apparatus comprising: a housing;a motor attached to said housing and transmitting an auxiliary steeringforce to a rotary shaft through a motor shaft; a worm formed on orfitted on said rotary shaft and having a gear portion formed of a metalor a resin; bearings provided in said housing, disposed respectively inpositions on both sides of said worm and rotatably supporting saidrotary shaft; an output shaft transmitting a steering force for steeringan axle and rotatably supported in a predetermined position of saidhousing; and a worm wheel formed on or fitted on said output shaft in away that meshes with said worm and having a gear portion formed of aresin, said motor including a worm gear mechanism installed in such aposition that a length which is the sum of a working radius of said wormand a working radius of said worm wheel becomes an axis-to-axis distancebetween said output shaft and said motor shaft, wherein elastic membersare disposed on both sides, in the axial direction, of said motor-sidedbearing adjacently to said bearing, said rotary shaft being set slightlymovable in the axial direction, said bearing positioned away from saidmotor is a rolling bearing, an elastic portion being provided on saidrotary shaft and biasing said worm in the meshing direction in parallelwith said rolling bearing, an outer ring of said bearing is fixedlyfitted in a cylindrical bearing holding member fixedly fitted in saidhousing, an inner ring of said bearing loosely receiving therein acylindrical buffer member fixedly fitted on said rotary shaft, and saidelastic portion is constructed of a biasing member rotatably supportingsaid rotary shaft, and of an elastic body receiving therein said biasingmember in a position eccentric in the meshing direction of said wormwith respect to an axis of said bearing, and fixed in the vicinity ofsaid bearing holding member.
 11. An electric power steering apparatusaccording to claim 10, wherein said buffer member is fixedly fitted onthe side of said inner ring of said rolling bearing, and looselyreceives therein said rotary shaft.
 12. An electric power steeringapparatus according to claim 10, wherein said biasing member and saidelastic body are integrally formed of a resin, and said biasing memberhas a plurality of holes formed along a periphery thereof to exhibitelasticity.
 13. An electric power steering apparatus according to claim10, wherein said elastic portion is constructed of said biasing member,a torsion spring wound on along a periphery of said biasing member, anda latching member resiliently latching both side end portions of saidtorsion spring and supporting said biasing member in a positioneccentric in the meshing direction of said worm with respect to the axisof said rolling bearing.
 14. An electric power steering apparatuscomprising: a housing; a motor attached to said housing and transmittingan auxiliary steering force to a rotary shaft through a motor shaft; aworm formed on or fitted on said rotary shaft and having a gear portionformed of a metal or a resin; rolling bearings provided in said housing,disposed respectively in positions on both sides of said worm androtatably supporting said rotary shaft; an output shaft transmitting asteering force for steering an axle and rotatably supported in apredetermined position of said housing; and a worm wheel formed on orfitted on said output shaft in a way that meshes with said worm andhaving a gear portion composed of the resin, wherein a pre-load applyingmechanism for applying a pre-load acting towards said worm wheel isprovided at the shaft end portion, distal from said motor, of saidrotary shaft.
 15. An electric power steering apparatus according toclaim 2, wherein said bearing is constructed of a first bearingpositioned in proximity to said motor and a second bearing positionedaway from said motor, and an axis-to-axis distance between said firstbearing and said rotary shaft is larger than an axis-to-axis distancebetween said second bearing and said rotary shaft.
 16. An electric powersteering apparatus according to claim 3, wherein said bearing isconstructed of a first bearing positioned in proximity to said motor anda second bearing positioned away from said motor, and an axis-to-axisdistance between said first bearing and said rotary shaft is larger thanan axis-to-axis distance between said second bearing and said rotaryshaft.
 17. An electric power steering apparatus according to claim 6,wherein said elastic member of said motor-sided bearing or said rotaryshaft in the vicinity of bearing positioned away from said motor or bothof said elastic member and said rotary shaft, is or are provided with astopper for controlling a quantity of movements of said rotary shaft inthe axial directions.
 18. An electric power steering apparatus accordingto claim 6, wherein said rotary shaft and said motor shaft arespline-connected to each other, and said rotary shaft is so formed as tobe swayable about the spline connecting portion serving as a fulcrum.19. An electric power steering apparatus according to claim 18, whereinsaid motor shaft is formed with a male spline portion, said rotary shaftis formed with a female spline portion, said motor shaft and said rotaryshaft are spline-connected to each other, and said male spline portionis formed in a drum-like shape so that its diameter is small at bothside ends thereof in the axial direction and is large at a centralportion thereof.